TDK Electronics · TDK Europe
Reliable temperature monitoring during the charging process for xEV batteries

In electric vehicles, temperature monitoring of all power components is a basic requirement for ensuring the safety, reliability and longevity of the entire system. Robust TDK and EPCOS NTC  temperature sensors available in a wide range of different designs are key components for this task.

The increasing trend towards xEV is forcing both the automotive industry and its suppliers to rise to new challenges. One particular challenge is the fact that the short charging processes involved in the charging of xEV batteries require a high charging output that is well into the three-figure kW range. This can lead to heating/overheating of the components involved in the charging process. Therefore, appropriate temperature monitoring of the charging station, the connector system, the busbars, the power electronics and the high-voltage battery is a basic requirement for ensuring that the charging process can be performed safely and efficiently. Insufficient temperature monitoring can lead to wear of the system components, reduced service life, or even failure of the components. In a worst-case scenario, major overheating can cause the battery to catch fire.

TDK has developed a range of special NTC temperature sensors for temperature monitoring in E-mobility applications. These sensors enable outstanding temperature monitoring thanks to their high levels of reliability and precision. This makes it possible to optimize the charging process, to increase the life time of the components subjected to thermal loads and implement safety-related functions. These NTC temperature sensors also stand out thanks to a high measurement accuracy and short response times. They are ideally suited to the monitoring of thermal processes involved in the charging process. 

The benefits at a glance

1 – Improved battery service life thanks to temperature monitoring

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Figure 1: The EPCOS NTC battery sensor stands out thanks to its long-term resistance to moisture. It is also easy to mount and can be adapted to specific customer requirements.

High-voltage batteries achieve optimum energy efficiency at precisely defined operating temperatures. Reliable monitoring and control of the battery temperature prevent the battery from overheating, which improves the battery life time and increases safety. To do this, the battery temperature must be measured at multiple points to prevent local overheating. A newly developed, special EPCOS screw-on temperature sensor is optimized for demanding requirements such as moisture, condensation and mechanical stress and approved for use in series vehicles. This sensor (Figure 1) consists of a robust, moisture-resistant housing, which contains an embedded NTC element. The sensor is simple to mount and to fixon the battery surface using the metal eyelet. It can also be mounted automated by robots. In the standard version, the screw-on sensor is designed to measure the battery temperature with a resistance value of 10 kΩ at 25 °C. This resistance value and the characteristic curve of the NTC temperature sensor can be adjusted according to customer-specific requirements. The standard measuring range is between -40 °C and +85 °C.

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2 – Monitoring of cooling media is a must

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Figure 2: Pipe-mounted sensors from TDK are suitable for different pipe diameters, are easy to mount and have short response times.

The temperature of the coolant can be used to determine the operating state of the battery. This temperature can be reliably measured using pipe-mounted sensors, which are also known as ‘clip-on’ sensors. These sensors are mounted on the coolant inlet and outlet. Because the sensors are mounted on the outside of the coolant pipes, no further sealing is required to protect them against the coolant leakage. This also means that there is more flexibility in terms of the design of the piping and installation of the sensor when compared with a solution with a defined mounting hole. TDK has a wide range of solutions for which the geometries and electrical parameters can be adjusted tocustomer-specific requirements.

What makes these newly developed NTC pipe-mounted sensors special is the combination of the sensor with a fixing element. This means that the sensor can be mounted on a pipe in a way that ensures the excellent thermal interconnection and at the same timethe sensor is secure and resistant to vibrations. Mounting and removing the sensor from the coolant pipe is quick and requires minimal effort. Thanks to the modular design of the sensor, it can be mounted on pipes with different diameters (Figure 2).

In order to ensure the long-term stability of the sensor system, a lot of attention was paied on the used materials. The clip material is compatible with the pipe materials. The sensor features a sealed metal sleeve that is positioned around the metal pipe of the cooling circuit and matches the thermal and electrical properties of the NTC element encapsulated within the metal sleeve to ensure short response times.

The resistance of the sensor system against frost, high temperatures and high humidity levels is also a key area of focus. When temperatures drop, condensation can form on the vehicle, and therefore also on the sensor. In this design, the sensor is encapsulated with plastic to provide suitable protection against condensation. The sensor can also be removed and re-mounted on the pipe to perform maintenance work more easily.

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3 – High-voltage-resistant busbar sensors for the power electronics

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Figure 3: The new TDK busbar sensor stands out thanks to its long-term stability and a high electrical resistance class.

Direct temperature measurement on the busbars of electric vehicles enables energy-efficient control and helps to prevent peak operating loads, which can reduce the vehicle’s service life. This challenging task requires sensors that can withstand high voltages and measure temperatures with great precision. Many of the sensors available on the market only specify the as-delivered condition of the sensors. However, in many cases this is insufficient, because key performance parameters can deteriorate over the sensor life time. In order to prevent damage to the control units, the high voltage strength of the sensor must also be maintained over the vehicle life time.

A new sensor with a high level of long-term stability has been designed specifically for mounting on busbars (Figure 3). The TDK NTC sensor is designed for a temperature range of -40 °C to +150 °C, whereby a short-term load of up to 200 °C is permissible. At 25 °C, the nominal resistance is 10 kΩ, with a B25/100 value of 3625 K and a tolerance of ±1 percent.

The sensor has been subjected to climatic, chemical and mechanical testing in accordance with the LV 124 life time tests and achieved electrical resistance class H3 according to LV 123, which corresponds to 2.5 kV DC. 

The connecting cables of the new temperature sensor meet the LV 112-4 standard for electrical cables in automotive vehicles and are twisted to improve EMC performance. The sensor includes an M4 eyelet made from copper alloy for mounting purposes. This choice of material offers good thermal interconnection and great material compatibility with the copper busbar, which prevents contact corrosion.

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4 – Safe monitoring of connector systems

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Figure 4: Thanks to its flat design, the TDK NTC temperature sensor of the NTCRP series is very well suited for integration within a connector system.

The connector systems between the charging station and the vehicle are also subject to high temperatures during the charging process. To ensure reliable temperature monitoring and to avoid overheating, the International Electrotechnical Commission stipulates that temperature sensors used in connector systems must meet the minimum requirements laid out in the IEC TS 62196-3 1 DIN standard. For this reason, reliable measuring accuracy with narrow temperature tolerances are essential requirements for a temperature sensor. The TDK NTCRP series has been developed with a miniaturised design for this application (Figure 4). The flat sensor housing enables optimal connection of the sensor to the surface of the connector contacts and is designed to withstand temperatures of up to 200 °C. 

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5 – Long-term stability busbar sensors for power electronics

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Figure 5: TDK B57703M NTC sensor for temperature compensation and temperature measurement with an incredibly high level of long-term stability.

Fast charging is considered to be one of the key factors in accelerating the success of the electric vehicle and enabling such vehicles to be used for longer journeys without the need to make charging stops that last several hours. With fast charging, the charging time can be reduced by several hours depending on the charging technology, battery chemistry and temperature – resulting in a charging time of between 15 and 30 minutes. However, very short charging times require very high currents, which places a high level of stress on the power electronics in the charging stations. This means that thermal monitoring is crucial.

A new sensor with a high level of long-term stability has been specifically designed for screw-on mounting on heat sinks and housings of systems in the power electronics (Figure 5). The TDK M703 NTC sensor is designed for a temperature range of -55 °C to +155 °C. At 25 °C, the nominal resistance is 5 kΩ with a B25/100 value of 3964 K. Alternative resistance values, nominal temperatures, resistance tolerances, cable lengths and AWG-28 wires are available upon request. The B57703M series features a coating material that is flame retardant according to UL 94 V-0. 

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In principle, one of the most important factors for further development of electric mobility is increasing energy efficiency. In this respect, the NTC temperature sensors from the extensive TDK product range provide a substantial contribution to the further development of electric mobility by enabling precise temperature measurements with long-term stability. TDK supports its customers with robust NTC temperature sensors and many years of experience in the development of these components. As a result, even the high requirements for reliable temperature monitoring of the entire battery charging process can be met.